(1) Field of the Invention
The present invention relates to an image forming apparatus such as a digital copier, facsimile machine, page printer and the like, in particular relating to an image forming apparatus which forms images by causing developer particles to jump to the recording medium.
(2) Description of the Prior Art
Among image forming apparatuses for outputting image data as a visual image on recording medium such as recording paper etc., one type is known which directly forms a toner image on the recording medium by making toner, the developer, jump onto the recording medium, as has been disclosed in Japanese Patent Application Laid-Open Hei 6 No. 155,798. As shown in FIG. 1, the image forming apparatus includes an image forming unit 151 having a toner supplying section 152 and a printing section 153. In this apparatus, toner 171 is made to jump from toner supplying section 152 and adhere to a sheet of paper 155, the recording medium. During this, the jumping of toner 171 is controlled in accordance with the image data.
Toner supplying section 152 is composed of a toner reservoir 170 for holding toner 171 as negatively charged developer particles, and a toner support 172 which supports toner 171 on its peripheral surface by magnetic force whilst rotating in the direction of arrow E. Printing section 153 is composed of an opposing electrode 175 of a cylindrical shape and a control electrode 176 which is provided between opposing electrode 175 and toner support 172. Opposing electrode 175 rotates in the direction of arrow F so that paper 155 is conveyed between opposing electrode 175 and control electrode 176 in the direction of arrow G.
As shown in FIG. 2, control electrode 176 has a plurality of gates 179 formed therein, each gate 179 having an annular electrode 177 formed around the edge thereof. As a voltage from a control power source 181 shown in FIG. 1 is selectively applied to these annular electrodes 177 in accordance with the image data, toner 171 supported on the peripheral surface of toner support 172 is made to jump toward opposing electrode 175 and pass through selective gates 179 hence being made to adhere to paper 155 which is placed between opposing electrode 175 and control electrode 176.
The image forming apparatus configured as above is one which directly forms the image on the surface of recording medium such as paper etc. Therefore, it is no longer necessary to use a developer medium such as a photoreceptor etc., which was used in conventional image forming apparatuses. Further, the step for transferring the image from the developer medium to the paper can be omitted, thus making it possible to eliminate degradation of the image due to this operation. Moreover, the structure of the apparatus can be simplified needing fewer parts, thus making it possible to reduce the apparatus in size and cost.
Since, in the image forming apparatus directly forming the image by making the toner jump as described above, the toner is selectively made to jump to form an image by controlling the voltage of the control electrode, the controllability of the voltage in the control electrode has a critical influence on the state of the image to be formed. The adherence of the toner bearing static charge is one of the typical causes that degrade the controllability of the voltage in the control electrode. More specifically, the potential of the control electrode on the basis of the toner supported on the toner support varies to a potential different from that applied to the control electrode for image forming, due to the potential which is derived from the static charge on the toner adhering to the control electrode, with respect to the control electrode. For example, if a voltage that allows the passage of toner is applied to the control electrode, the actual voltage may vary or be close to a potential that prohibits the passage of toner, due to the static charge on the toner adhering to the control electrode, and hence no or only an insufficient amount of toner will transfer from the toner support to the paper. Resultantly, the condition of the image formed will be badly deteriorated with image defects, print failure, low contrast and reproduction failure of halftone images.
Adherence of the toner to the control electrode occurs on the surface thereof and the interior of the gates, due to electrically attractive force, typified by `image force`, as well as because the friction between the toner surface and the control electrode surface exceeds the electric force exerting on toner particles from the electric fields formed between the control electrode and the opposing electrode or formed between the control electrodes and the toner support. As a result, in order to remove the toner adhering on the control electrode, it is necessary to create a stronger electric field or urging force by a physical method.
In order to create a stronger electric field than that formed between the control electrode and the opposing electrode, it is necessary to apply a greater voltage to the opposing electrode or place the opposing electrode closer to the control electrode. However, creation of a stronger electric field between the control electrode and the opposing electrode requires additional components such as a high-voltage power source and its control circuits. On the other hand, an arrangement of placing the opposing electrode closer to the control electrode requires a device for shifting the opposing electrode toward the control electrode. In either case, it is impossible to avoid sharp increase in the size and cost of the apparatus due to an increased number of parts.
Because of the above reasons, Japanese Patent Application Laid-Open Hei 6 No. 218,981 discloses a configuration in which a bar having an opposing electrode at one end thereof is provided rotatably at the middle part thereof and the other end has a brush roller or scraper to be abutted against the surface of the control electrode. Other configurations than this, are also disclosed such as that having an adhesive tape selectively contacting the surface of the control electrode, that having an air stream generating means for blowing an air stream onto the surface of the control electrode, and that having a vibrating means for vibrating the control electrode.
However, any of the configurations disclosed in Japanese Patent Application Laid-Open Hei 6 No. 218,981 cannot avoid the problem of increase in the size and cost of the apparatus due to an increased number of parts. Further, when adhering toner is removed from the control electrode by blowing an air stream over the surface of the control electrode or vibrating the control electrode, an extensive area other than the control electrode inside the apparatus will be stained with toner.